1. Field of the Invention
This invention relates to an improved driving mechanism for transmitting driving force derived from a single power source to a plurality of objects to be driven by the force by a single timing belt. It also relates to an improved image forming apparatus comprising a driving mechanism for transmitting driving force derived from a single power source to a photosensitive drum and image forming means provided around the photosensitive drum by way of a single timing belt.
2. Description of the Related Art
In a conventional image forming apparatus, such as a copying machine or a laser printer or in an automated office machine, a number of components of the apparatus are normally and simultaneously driven by driving force derived from a single power source and transmitted by way of power transmitting means selected from gears, pulleys, chains, wires, timing belts and other elements. Of these power transmitting elements, a chain generates a large noise when it is engaged with a sprocket wheel while a wire slips on pulleys supporting them if the tension applied to it is inadequate, and can be prematurely broken if the abrasion of its surface is accelerated by the slipping action. In order to avoid these problems, there has been proposed a driving mechanism in which a timing belt made of a rubber material such as chloroprene rubber is used in place of a chain or a wire and one or both surfaces of the timing belt is covered with woven nylon cloth which is firmly bonded thereto, a number of teeth being formed thereon for engagement with toothed belt pulleys. A driving mechanism using a timing belt of this type generates only relatively small noises as compared with a driving mechanism using a wire or a chain, and the timing belt does not slip on the toothed belt pulleys.
However, a timing belt on both surfaces of which teeth are formed to engage toothed belt pulleys, as described above, is not free from generating noises on both surfaces of the belt as it abuts the toothed peripheries of the belt pulleys. The noises from the both surfaces of the belt have a same frequency because the pitch of the teeth on one surface of the belt is identical with that of the teeth formed on the other surface. For instance, if the pitch of teeth on each surface of the timing belt is P [mm] and the number of teeth is Z, and if the number of revolutions of each belt pulley is N [rpm], a noise with a frequency of ZN/60 [Hz] is generated from each surface as the teeth on the each surface of the belt abut against the toothed peripheries of the each belt pulley. The teeth abutting noises generated on both surfaces of the belt with an identical frequency are added together so that the peak level of the combined noises becomes larger than that of each sound. Thus, the use of a driving mechanism comprising a timing belt of this type in an office machine such as an image forming apparatus will be inadequate in office areas where such noises can become a nuisance.